Chitinases (EC 3.2.1.14), enzymes catalyzing hydrolysis of chitin, exist in various organisms including viruses, bacteria, fungi, insects, plants, and animals. Chitin, a linear β-1,4-linked homopolymer of N-acetyl-D-glucosamine (GlcNAc) is widely distributed in fungal cell walls, the exoskeleton of arthropods, and the outer shell of crustaceans. Chitinolytic activity is involved in food digestion, morphogenesis, and degradation of cuticle. In higher plants that lack endogenous substrates, chitinases are involved in the defense against pathogens and function as pathogenesis-related (PR) proteins (Collinge et al. 1993, Plant J. 3, 31-40.3). Among various PR-proteins, chitinases and β-glucanases hydrolyze chitin and β-1-3-glucan, which are the major structural components in the cell wall of many phytopathogeneic fungi (Selitrennikoff, 2001, Appl. Environ. Microbiol. 67, 2883-2894). Chitinases, like β-glucanases, contribute to plant defenses due to their ability to degrade fungal pathogens cell wall, and thereby inhibit their growth. In the early stage of pathogenesis, apoplastic chitinases release elicitor molecules that activate the defense mechanisms of plants. Then the synthesis and secretion of apoplastic chitinases occur to enhance the infection signaling, while the vacuolar chitinase degrades the fungal cell wall to inhibit pathogen growth (Collinge et al. 1993, Plant J. 3, 31-40). Chitinases have been used in agriculture to control plant pathogens (Broglie et al., 1989, Plant Cell 1, 599-607.)
Based on similarity in the catalytic domains of glycosyl hydrolases, chitinases have been classified into families 18 and 19 (van Aalten et al., 2001, Proc. Natl. Acad. Sci. USA 98, 8979-8984). Chitinases can also be divided into two categories based on function: exochitinases and endochitinases. Exochitinases specifically hydrolyze the β-1,4-glycoside linkages at the non-reducing end of the chitin chain, whereas endochitinases cleave the internal linkages. Some plant endochitinases have lysozyme activity and hydrolyze the β-1,4-linkages between N-acetylmuramic acid and GlcNAc residues in peptidoglucan (Subroto et al., 1999, J. Mol. Evol. 49, 819-821. Transgenic Res. 15, 337-347).